1. Field of the Invention
This invention relates to a novel method of preparing metal-containing zeolite catalysts and to metal-containing zeolite catalysts of increased stability and activity prepared thereby.
2. Description of the Prior Art
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic capabilities for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure within which there are a large number of small cavities which are interconnected by a number of still smaller channels. These cavities and channels are precisely uniform in size. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties.
Such molecular sieves include a wide variety of positive ion-containing crystalline aluminosilicates, both natural and synthetic. These aluminosilicates can be described as a rigid 3-dimensional network of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen is 1:2. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example, an alkali metal or an alkali earth metal cation. This can be expressed by the formula wherein the ratio of Al to the number of the various cations such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation can be exchanged either entirely or partially by another type of cation using ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the size of the pores in the given aluminosilicate by suitable selection of the particular cation.
The catalytic properties of metal-loaded aluminosilicates as is well known, have been demonstrated to be extremely important to petroleum, chemical and enzymatic reactions. The aluminosilicates have been activated, i.e., metal loaded for these reactions by methods employing impregnation, vapor deposition and base-exchange of the desired metal to be loaded. The use of aqueous solutions, especially those of polyvalent metal salts, is the standard method employed to exchange the metals into the crystalline aluminosilicate structure. The resulting wet metal-containing crystalline aluminosilicate zeolite is thereafter dried and subsequently subjected to a thermal treatment. The finished catalyst contains the metal component distributed in the zeolitic structure in metallic form.
Zeolites which have been loaded with metals according to the above-described conventional method are subject to substantial limitations. In particular, the metals incorporated in such zeolites may not be adequately anchored within the zeolite channels. Under the severe conditions of temperature and pressure encountered in catalysis, the incorporated metals migrate out of the pores of the zeolite to the zeolite surface. A reducing atmosphere and the presence of hydrocarbon exacerbates this migration.
Supported metal atoms are known to be bound to the support by Van der Waal's force, approximately 5 Kcal/g atoms. Increasing the number of metal atoms in a cluster results in an increase in the bonding energy of the metals to the support. Thus, metal atoms migrate easily to form clusters, especially at elevated temperatures when the mobility of these atoms is especially high.
Metal migration can be reduced by increasing the energy of interaction between the metal and the support. For example, it has been demonstrated that by using low valent metals, e.g. Pb or Sn, or early transition metals, e.g. Mo, W, Re, as a stabilizing center or anchored center or amorphous oxide supports such as SiO.sub.2 and Al.sub.2 O.sub.3, the binding energy between the metal atoms and the support, through the stabilizing center can be increased. An example of such a catalyst system can be represented as follows: ##STR1## wherein Mo is the stabilizing center and M is the metal or metals being supported. It has been shown that metal to metal binding in bi-nuclear organometallic complexes can reach as high as 80 Kcal/mole.
The amorphous oxide supported metal catalysts with anchored center have also been shown to provide better metal dispersity and activity than the conventional metal-exchanged catalysts. For example, the activity for benzene hydrogenation was increased by 10 fold when Re or Mo was used as an anchored center for platinum metal. A more dramatic increase of 10.sup.4 fold in Pt activity for ethane hydrogenalysis was obtained with W as an anchored or stabilizing center on a SiO.sub.2 support. Organometallic complexes, such as allyl complexes have been utilized to introduce both the stabilizing center and the catalytically active metal or metals within the support.